1. Field of the Invention
This invention relates to a semiconductor device.
2. Background Art
The on-resistance of a vertical power MOSFET (metal-oxide-semiconductor field effect transistor) greatly depends on the electric resistance of its conduction layer (drift layer). The dopant concentration that determines the electric resistance of the drift layer cannot exceed a maximum limit, which depends on the breakdown voltage of the p-n junction formed between the base region and the drift layer. Thus there is a tradeoff between the device breakdown voltage and the on-resistance. Improving this tradeoff is important for low power consumption devices. This tradeoff has a limit determined by the device material. Overcoming this limit is the way to realizing devices with low on-resistance beyond existing power devices.
As an example MOSFET to solve this problem, a structure with p-type pillar layers and n-type pillar layers provided in the drift layer is known as a superjunction structure. In the superjunction structure, a non-doped layer is artificially produced by equalizing the amount of charge (amount of impurity) contained in the p-type pillar layer with that contained in the n-type pillar layer. Thus, while holding high breakdown voltage, a current is passed through the highly doped n-type pillar layer. Hence, a low on-resistance beyond the material limit is realized. In order to hold high breakdown voltage, the amount of impurity in the n-type pillar layer and the p-type pillar layer needs to be accurately controlled.
Such a MOSFET having a superjunction structure in the drift layer is different also in the design of its termination structure from normal power MOSFETs. Like the cell section, the termination section also needs to hold high breakdown voltage. In the case where the termination section also includes a superjunction structure, variation in the amount of impurity also results in decreased termination breakdown voltage, and hence decreased device breakdown voltage. To solve this, in a structure disclosed in JP-A 2000-277726(Kokai), for example, the termination section is formed from a high-resistance layer without including a superjunction structure to increase the termination breakdown voltage.
However, the termination formed from a high-resistance layer has a low impurity concentration, and hence the depletion layer is easy to extend therein. Thus, the termination distance needs to be lengthened so that the depletion layer does not reach the dicing line. Comparing at the same chip size, a longer termination distance results in decreasing the effective area ratio of the chip and increasing the on-resistance of the chip. To realize the same chip on-resistance with the long termination distance left unchanged, the chip size needs to be increased. This decreases the number of chips that can be formed in one wafer and increases the chip cost.
Furthermore, in this structure, the superjunction structure is discontinuous between the cell section and the termination section. Upon application of high voltage, the depletion layer does not extend from the high-resistance layer to the superjunction structure. Hence, the impurity concentration in the p-type pillar layer or the n-type pillar layer at the outermost portion of the cell section superjunction structure, where the discontinuity exists, needs to be approximately half that in the cell section. Concentration variation in the outermost pillar layer results in decreased breakdown voltage, like variation in the amount of impurity in the pillar layer of the cell section. Hence, the amount of impurity requires the same controllability as in the other pillar layers.